1. Field of the Invention
The present invention relates to an electronic diagnostic system for making a diagnosis on deterioration or failure in a so-called heated type oxygen sensor for automotive engines, and specifically to technologies for diagnosing deterioration or failure in the heated type exhaust oxygen sensor only when the sensing element of the heated type oxygen sensor is fully activated and thus reaches operating temperature.
2. Description of the Prior Art
Today's automotive engines employ an exhaust oxygen sensor, simply called an O.sub.2 sensor, which is located in either the engine exhaust manifold or exhaust piping to monitor the percentage of oxygen contained within the exhaust gases. As is well-known, the oxygen sensor is used as a feedback element in closed-loop engine control systems in which an air-fuel mixture ratio based on the oxygen sensor's signal is maintained at as close to stoichiometric as possible. On earlier model of cars, an ECM or ECU often includes an oxygen sensor failure diagnostic system for diagnosing deterioration of the oxygen sensor by comparison between a frequency of the oxygen sensor's signal and a predetermined reference frequency. One such O.sub.2 sensor failure diagnostic system has been disclosed in Japanese Patent Provisional Publication No. 2-204648. Two types of oxygen sensors, namely an unheated type of O.sub.2 sensor and a heated type of O.sub.2 sensor, are in wide use on today's automotive vehicles employing an electronic control module (ECM) or an electronic engine control unit (ECU). The unheated type of oxygen sensor will not output a voltage signal to the on-board ECU until the sensing element has reached operating temperature. The engine is first started, the engine operates in a so-called open-loop condition (an open-loop mode), since the ECM ignores any voltage signals from the unheated oxygen sensor, and thus the engine operates in a preprogrammed ECU sequence. On the other hand, the heated type oxygen sensor is fully operational within 10 seconds of engine startup, regardless of the exhaust gas temperatures. In this manner, since the heated type oxygen sensor operates almost immediately, the engine can enter a so-called closed-loop condition (a closed-loop mode) quickly, allowing the ECM to maintain the engine air-fuel-mixture ratio at stoichiometric almost as soon as the engine is started. When the sensing element has not yet reached its operating temperature with the oxygen sensor's heater energized or activated at initial engine startup, the sensing element is in its in-active state. At this time, the diagnostic system cannot make an accurate diagnosis on deterioration of the oxygen sensor. For the reasons set out above, a conventional diagnostic system for a heated type oxygen sensor is designed to initiate to diagnose deterioration of the oxygen sensor when a predetermined elapsed time (hereinafter referred to simply as a "delay time"), measured from the beginning of activation of the oxygen sensor's heater (for example, from engine startup), has been reached. As seen in FIG. 5, the heater of the heated type oxygen sensor is generally de-energized or turned off in a specified high engine speed range and/or a specified high engine load range. A zone (as indicated by a blank area in FIG. 5) in which the heater is de-energized or turned off will be hereinafter referred to as a "heater de-energized zone" or "heater de-activated zone", whereas a zone (as indicated by a shadow area in FIG. 5) in which the heater is energized or turned on will be hereinafter referred to as a "heater energized zone" or "heater activated zone". In FIG. 5, a substantially central rectangular zone, which is defined by both a predetermined engine speed range and a predetermined engine load range, corresponds to a diagnostic permissible zone (simply diagnostic zone). In the prior art diagnostic system, even when there is a transition from the heater de-energized zone to the diagnostic permissible zone and thus the oxygen sensor has been already fully warmed up and thus reached its operating temperature, the prior art system is designed to begin to make a diagnosis on deterioration of the oxygen sensor from the time when the previously-noted predetermined delay time has been elapsed, in the same manner as a period of engine startup. As set forth above, in the prior art system, the predetermined delay time necessary for initiation of a desired diagnostic process for deterioration of the oxygen sensor, is fixed to a predetermined constant time, regardless of when the engine is started from cold or when the engine is fully warmed up.